The dependence of the elastic properties of osteoporotic cancellous bone on volume fraction and fabric

Abstract

Osteoporosis is a progressive systemic skeletal condition characterized by low bone mass and microarchitectural deterioration, with a consequent increase in susceptibility to fracture. Hence, osteoporosis would be best diagnosed by in vivo measurements of bone strength. As this is not clinically feasible, our goal is to estimate bone strength through the assessment of elastic properties, which are highly correlated to strength. Previously established relations between morphological parameters (volume fraction and fabric) and elastic constants could be applied to estimate cancellous bone stiffness in vivo. However, these relations were determined for normal cancellous bone. Cancellous bone from osteoporotic patients may require different relations. In this study we set out to answer two questions. First, can the elastic properties of osteoporotic cancellous bone be estimated from morphological parameters? Second, do the relations between morphological parameters and elastic constants, determined for normal bone, apply to osteoporotic bone as well? To answer these questions we used cancellous bone cubes from femoral heads of patients with (n=26) and without (n=32) hip fractures. The elastic properties of the cubes were determined using micro-finite element analysis, assuming equal tissue moduli for all specimens. The morphological parameters were determined using microcomputed tomography. Our results showed that, for equal tissue properties, the elastic properties of cancellous bone from fracture patients could indeed be estimated from morphological parameters. The morphology-based relations used to estimate the elastic properties of cancellous bone are not different for women with or without fractures.

Introduction

Osteoporosis is a progressive systemic skeletal condition characterized by low bone mass and microarchitectural deterioration, with a consequent increase in susceptibility to fracture (Conference Report, 1993). Hence, osteoporosis would be best diagnosed by in vivo measurements of bone strength. As this is not clinically feasible, our goal is to estimate bone strength through the assessment of its elastic properties, which are highly correlated to strength (R²⩾0.95, Hodgskinson and Currey, 1993; Goulet et al., 1994; Hou et al., 1998).

The apparent elastic properties of cancellous bone depend on bone density (volume fraction), cancellous architecture, and the tissue modulus. Experimental studies have revealed that bone density or volume fraction alone can explain 70–80% of the variance in mechanical properties (Keaveny et al., 1994; Goulet et al., 1994; Hou et al., 1998). Although osteoporosis is characterized by low bone density, it cannot clearly discriminate between bone from individuals with and without fractures (Melton et al., 1989). As osteoporosis is also characterized by deterioration of the cancellous architecture, it may be helpful to estimate the mechanical properties not only through bone density, but also through cancellous architecture. Incorporation of the tissue modulus, although not affected by osteoporosis (Homminga et al., 2002), may further improve the estimate. A good estimate of the mechanical properties that incorporates both bone density and cancellous bone architecture has been published by Cowin.

In 1985, Cowin proposed relations between the morphological parameters (volume fraction and fabric), on the one hand, and the apparent elastic properties on the other. These relations were later shown to explain over 90% of the variance in the apparent elastic properties (Turner et al., 1990; van-Rietbergen et al., 1998; Kabel et al., 1999b). These relations between morphological parameters and elastic properties could, in theory, be applied to estimate cancellous bone stiffness in vivo. However, none of these relations were determined for cancellous bone from patients with osteoporotic fractures. Different relations may be needed for cancellous bone from these patients.

This raises two questions. First, can the elastic properties of osteoporotic cancellous bone be estimated from morphological parameters? Second, do the relations between morphological parameters and elastic constants, that were determined for normal bone, apply to osteoporotic bone as well?.